Genotype = the genes of an organism; for one specific trait we use two letters to represent the genotype. A capital letter represents the dominant form of a gene (allele), and a lowercase letter is the abbreviation for the recessive form of the gene (allele).
Phenotype = the physical appearance of a trait in an organism
For example, Horse Color, black coat is the dominant trait and red coat is recessive.
Since the "Black-coat code" and the "Red-coat code" are alleles (two forms of the same gene), we abbreviate them with two forms of the same letter. So we use "E" for the dominant allele/trait (black coat) and "e" for the recessive allele/trait (red coat).
Our possible genotypes & phenotypes would be like so:
Symbol Genotype Name Phenotype
EE homozygous (pure) Black coat
dominant
Ee heterozygous (hybrid) Black coat
ee homozygous (pure)
recessive Red coat
PUNNETT SQUARE
Here are the basic steps to using a Punnett Square when solving a genetics question. After you get good at this you should never miss a genetic question involving the cross of two organisms.
STEPS:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. draw a p-square
4. "split" the letters of the genotype for each parent & put them "outside" the p-square
5. determine the possible genotypes of the offspring by filling in the p-square
6. summarize results (genotypes & phenotypes of offspring)
Step #1: Determine the genotypes of the parent organisms.
Sometimes this already done in the question for you. If the question says "Cross two organims with the following genotype: Tt & tt", it's all right there in the question already.
More likely is a question like this: "Cross a solid color horse with one that is heterozygous for tobiano". Here, you have to use your understanding of the vocab to figure out what letters to use in the genotypes of the parents. Heterozygous always means one of each letter, so we'd use "Tt" (where "T" = tobiano, & "t" = solid). The only way for a horse to be solid color, is when it has 2 lowercase "t's", so that the solid parent is "tt". So the cross ends-up the same as in my first example: Tt x tt.
Now, we can make things just a little more tricky. Let's use horse color again in this example. Black is dominant (E), and Red is recessive (e). What if a question read like this: "Predict the offspring from the cross of a red horse and a black horse if the Black horse's Dam (mom) was red". Oooooh, is this a toughy? First things first: the only way for the red horse to be red (the recessive trait) is if it's genotype is homozygous recessive (2 little letters), so the red horse is "ee". Now, the black horse's genotype could be either "EE" or
"Ee". If its Dam was red (ee), then this black horse MUST have inherited a little "e" from its Dam. So the black one in our cross is "Ee" (not "EE"), and our Horse cross is: Ee x ee.
Step #2: Write down your "cross" (mating). Write the genotypes of the parents in the form of letters (ex: Tt x tt). Tobiano x Solid
Step #3: Draw a p-square
The basic naked p-square looks like a window pane :
Step #4: "Split" the letters of the genotype for each parent & put them "outside" the p-square.
For an example cross we'll use these parental genotypes: Tt x tt.
Take the genotype letters of one parent, split them and put them on the left, outside the rows of the p-square.
What we've done is taken the hetrozygous tobiano (Tt) and put its big "T" out in front of the top row, and the little "t" out in front of the bottom row. When we fill-in the p-square, we will copy these "tees" into each of the empty boxes to their right. So the big "T" will be in each of the boxes of the top row, and the lowercase "t" will be in the two boxes of the bottom row.
Now take the two letters of the second parent's genotype, split 'em up, and place them above each of the two columns of the p-square.
Now, when it comes time to filling things in, those lowercase "t's" will each be copied into the two boxes directly below them. So after the next step, each little box will have two letters in it (one "tee" from the left & one "tee" from the top). These new 2 letter combos represent possible genotypes of the offspring.
Step #5: Determine the possible genotypes of the offspring by filling in the p-square.
I kinda gave this away already, but to "determine the genotypes of the offspring" all we gotta do is fill-in the the boxes of the p-square. Again we do this be taking a letter from the left & matching it with a letter from the top. Like so:
1. Filling in the top-left box:
2. Filling in the bottom-left box:
4. Filling in the top-right box:
3. Filling in the bottom-right box:
"One from the left, one from the top... one from the left, one from the top...one from the left, one from the top...one from the left, one from the top".
Step #6: Summarize the results (genotypes & phenotypes of offspring).
Simply report what you came up with. You should always have two letters in each of the four boxes.
In this example, where our parent Horse's were Tt (tobiano) x tt (solid), we get 2 of our 4 boxes with "Tt", and 2 of our 4 with "tt". The offspring that are "Tt" would end up with the tobiano pattern (the dominant trait) and the
"tt" horse's would have a solid coat (the recessive trait).
So our summary would be something like this:
Parents Horses
("P" Generation)
Offspring
("F1" Generation)
Genotypes:
Tt x tt
Phenotypes:
Tobiano x
Solid
Genotypes:
50% (2/4) Tt
50% (2/4)
tt
Phenotypes:
50% Tobiano
50%
Solid
scientific-note:
You know how, in Step #4, when we "split" the letters of the genotype & put them outside the p-square? What that step illustrates is the process of gametogenesis (the production of sex cells, egg & sperm). Gametogenesis is a cell division thing (also called meiosis) that divides an organism's chromosome number in half. For example, in humans, body cells have 46 chromosomes a piece. However, when sperm or eggs are produced (by gametogenesis/meiosis) they get only 23 chromosomes each. This makes sense (believe it or not), because now, when the sperm & egg fuse at fertilization, the new cell formed (called a zygote) will have 23 + 23 = 46 chromosomes.
So, when the chromosome number is split in half, all of the two letter genotypes for every trait of that person (or organism) get separated. Which is why we do what we do in Step #4.
